1. Field of Invention
This invention relates to energy conservative current supplies, and more particularly to energy conservative current sources adapted for rapid response to variable demand.
2. Description of the Prior Art
The use of magnetic deflection in cathode ray display systems of many types is well known. One reason for preferring magnetic deflection is the superior brightness and resolution characteristics which may be obtained. However, magnetic deflection systems consume considerably more power than do electrostatic systems. The current provided to a deflecting yoke associated with a CRT must normally vary from some negative value (for deflection to one edge of the screen), through zero (for deflection at the center of the screen) to a high positive value (for deflection at the opposite edge of the screen). Since the deflection must be in accordance with the desired picture, it must be provided by a linear amplifier working with suitable positive and negative voltage supplies. If deflection is to change extremely rapidly, then the power supplies must additionally be of relatively high voltage. But when the rate of change of current to the yoke is relatively low, then the driving voltage must be relatively low. The output amplifier for driving a yoke must therefore drop considerable voltage over a considerable portion of the time while supplying substantial current. This is what consumes the power. In a resonant display (such as a TV or other raster-type system) the energy is circulated in reactive elements and conserved. But in other displays (such as stroke-written displays), this is not possible.
To conserve energy in non-resonant displays, the use of energy-conservative, modulated power supplies is known. These conserve energy by duty-cycle modulation of a current supplied to the load. Such power supplies are either full-on or full-off. When full-on, they are like a switch which is closed and makes a low resistance connection, so that the passage of a large current therethrough does not dissipate much power. When they are full-off, there is no current flow so there can be no power dissipation. By causing the power supply to be on for the correct percentage of the time, at a fairly high switching rate or frequency, the average current can be controlled with relatively small power losses within the power supply itself. However, devices of this type have not been provided with adequate control (in terms of a faithful, linear current representation of an input control signal) for high quality CRT display systems.